This application incorporates by reference Taiwanese application Serial No. 88121612, filed on Dec. 9, 1999.
1. Field of the Invention
The invention relates in general to circuitry for minimal phase delay and zero crossing filtering and more particularly to circuitry for minimal phase delay and zero crossing filtering, which is used for high-speed glitch-free analog-to-digital signal conversion.
2. Description of the Related Art
Zero-crossing (ZC) filters are widely applied in circuit designs, such as the circuitry design for converting analog to digital signals.
Referring now to FIG. 1, it illustrates a conventional zero crossing filter having two inputs, an input signal V.sub.I and a reference voltage V.sub.ref, and outputting an output signal V.sub.O '. FIG. 2 depicts the transfer characteristic of the zero crossing filter in FIG. 1. FIG. 2 indicates that when V.sub.I becomes lower than V.sub.ref, V.sub.O ' changes from the high level state L.sub.1 to the low level state L.sub.0 ; otherwise, when V.sub.I becomes higher than V.sub.ref, V.sub.O ' changes from the low level state L.sub.0 to the high level state L.sub.1.
FIG. 3 contains the waveform diagrams of the input and output signals of the filter in FIG. 1. In the case of V.sub.ref of 0 voltage, the ideal output signal waveform of the filter is the signal V.sub.O. In FIG. 3, there are four zero crossing points indicated by the intersections of the vertical dotted lines and the time axes. However, the input signal V.sub.I may be interfered with by some external high frequency noise, resulting in undesired glitches near the zero crossing points in the output signal waveform, such as the waveform of the output signal V.sub.O ' shown in FIG. 3. The glitches negatively affect the quality of the output signal V.sub.O, which will easily cause the circuitry for processing the output V.sub.O ' to operate improperly.
For eliminating glitches, there are two conventional approaches. In the first approach a hysteresis zero crossing filter is applied. Referring now to FIG. 4, it illustrates a hysteresis zero crossing filter having two input signals, V.sub.I and V.sub.ref, and an output signal V.sub.O. FIG. 5 depicts the transfer characteristic of the hysteresis zero crossing filter in FIG. 4. According to FIG. 5, when V.sub.I exceeds the high threshold voltage V.sub.H, output signal V.sub.O changes from the low level state L.sub.0 to the high level state L.sub.1 ; on the other hand, when V.sub.I becomes below the low threshold voltage V.sub.L, V.sub.O changes from the high level state L.sub.1 to the low level state L.sub.0.
Referring to FIG. 6, it contains the waveform diagrams of the input and output signals of the hysteresis zero crossing filter. Because of the hysteresis of the filter, the output signal V.sub.O does not contain any glitch. However, due to the hysteresis, a phase delay of the output signal V.sub.O occurs.
Another approach to diminishing glitches is to cascade the zero crossing filter, in which glitches occur, with a digital logic circuitry for processing the output signal of the filter. The digital circuitry detects whether a glitch occurs on the output signal of the filter. If so, the circuitry temporarily stops sampling the output signal of the filter for a period of time, i.e. a response time delay, until the input signal V.sub.I is stable. Although, in this approach, glitches are reduced effectively, a phase delay of the output signal V.sub.O occurs also.
In systems having high-speed analog-to-digital signal conversion, phase delay in signals may affect the total performance of the systems. For instance, in a digital versatile disk (DVD) system, a servo control signal is used to control the actuator of the optical head for tracking and focusing. If the servo control signal has a phase delay, the response speed of the servo controller becomes slower such that the system fails to maintain the effective performance.